The first photo is our porch. I want to redo it and extend it out about 6 ft. Here's the problem. When we bought we were told the house was moved to the property.
These next 2 photos are the bottom of the porch. Yes when you stand on my porch, you stand over my basement. I have a feeling when they measured for the foundation, they didn't consider the porch.
My question. When I remove the existing structure, what do I do to stop air and water from coming in? I will be insulating but I need it sealed first.
Thanks

This should be built like a reguler floor for the house with slight slope to it covered with plywood t&g and covered with a waterproof decking like duradeck of fibreglass but either of those should run about 6" up the wall behind the siding. When you insulate for living space below there should be air gap between the insulation and the deck and that air should be vented to the outside. This usually done by running joists one way and the laying 2x4s on the flat the other way fo air gap. As you are very close to the ground you may want to use treated lumber foir the framing.
And welcome to the site.

Did you ever want outside access to your basement? Looks like you could do that pretty easy with how it's built. And that's a small porch around here we call that a farmers porch. You could add stairs from the window over and then extend a bigger porch out if you need the outdoor space. Just an idea.

It's pretty hard to figure out from the pictures, is there a barring wall under the side of the house or are the floor joist continuing under the floor of the house all in one. I had one of these to deal with, I re-built the floor and closed it in as part of the house.

I gather that the post and the outside edge of the porch sits on the block foundation. While your pictures don't really show it I wonder if the foundation was built because your "porch" wasn't really a porch originally as they are normally built on a house, rather it was a mud room that someone just decided to open to the outdoors.
From what I see, you should build a wall to close it in, ad a slider or french door, and instead of extending the "porch' and given you're a couple inches above grade, just add a patio. Either poured or patio block.